/Engine/Source/Editor/GraphEditor/Public/MarqueeOperation.h
Given a mouse event, figure out what the marquee selection should do based on the state of Shift and Ctrl keys
static [FMarqueeOperation::Type](API\Editor\GraphEditor\FMarqueeOperation\Type) OperationTypeFromMouseEvent ( const [FPointerEvent](API\Runtime\SlateCore\Input\FPointerEvent) & MouseEvent )